<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>邮箱</title>
</head>
<body>
            <script>
//                var s = /^([a-z]@[a-z].com|.com.cn)|([1-9]@[a-z].com|.cn)|([a-z][0-9]@qq.com)|([a-z]_[a-z]@qq.com)|([a-z].[a-z]@qq.com)$/;
//                var z = /^$/
var    s = /^[^0_\.][a-z0-9\._]*@([a-z0-9]+\.)+[a-z]{2,5}/;
                var h = 'frank@mail.sina.com.cn';
                var k = h.match(s);
                console.log(k)
            </script>
</body>
</html>
frank@qq.com
frank@sina.com
frank@sina.com.cn
frank@mail.sina.com.cn
730778@qq.com
frank123@qq.com
frank_chan@qq.com
frank.chan@qq.com